1. Field of the Invention
The invention relates to a method of synchronising the receiver arrangements in a digital multiplex transmission system.
2. Description of the Related Art
For the transmission of messages by a transmission means (for example lines, radio-channels) which is used in common by a plurality of subscribers, three basic methods are known, namely the code-division multiplex method, the frequency-division multiplex method and the time-division multiplex method.
In the code-division multiplex method the different messages conveyed through a common transmission means are, for example, modulated on a sub-carrier by basic modulation and the resultant signal, which in comparison with the channel bandwidth is a narrow-band signal, is spectrally distributed over the channel bandwidth by multiplex modulation with the aid of a code word characterising the receiver. The code-division multiplex channel (message transmission channel) thus obtained is not limited in time or bandwidth, but is limited relative to the power density. Reception of the signal is not effected by selection on a time or frequency basis but on the basis of the spectral coding. The plurality of spectrally encoded messages, superposed in the code-division multiplexed channel, are selected in the receiver on the basis of code words assigned thereto. For two-stage modulation (basic and multiplex modulation), phase shift keying (PSK) or frequency shift keying (FSK) are often utilized in radio transmission systems.
The first stage of the transmitter is supplied, for example, with a digitized speech signal (after having been converted in an analog-to-digital converter) and includes, for example, a multiplicative mixer. In the multiplicative mixer the supplied digitized speech signal is combined with a codeword assigned to this transmitter, which results in a spectral distribution. In a second modulation stage of the transmitter the wide-band signal (modulated, binary character sequence) is converted into a frequency band position suitable for transmission.
Recovering the message at the receiver is effected in the above-described code-division multiplex method by a sequence of basic demodulation and multiplex demodulation. Conversion to a frequency band position (for example baseband position) suitable for multiplex demodulation is effected in the basic demodulation stage by multiplying the signal by a reference sub-carrier. With the aid of a code word generator arranged in the receiver and also a code synchronising circuit the spectral distribution is cancelled, after the code word generator has been synchronised in the appropriate phase with the received code word. As a result thereof, the signal energy which has previously been spectrally distributed over the entire transmission band is compressed to the original frequency band, whilst the adjacent characters entering the receiver with a different multiplex modulation remain in the spectrally distributed state and can be suppressed by a bandpass filter having a bandwidth corresponding to the bandwidth of the non-distributed signal.
The system-determined residual interference in the multiplex demodulation, produced by the other signals, is lower according as the values of the cross-correlation functions between the different code words employed are lower and the distribution factor is larger. A non-zero value of the cross-correlation function reduces the signal-to-noise ratio. The signal-to-noise ratio and the synchronising period are determined by the cross-correlation and auto-correlation function.
In the frequency-division multiplex method the total bandwidth available for message transmission is subdivided into narrow frequency bands which each correspond to a message transmission channel. Such a narrow frequency band is available to the subscriber for the duration of the radio transmission.
In the time-division multiplex method each subscriber has at its disposal of the total bandwidth of a single radio channel which the subscriber may only utilize for short periods of time. The characters or character sequences of different subscribers are interleaved and are transmitted with correspondingly higher bit rates through the single radio channel, each time a channel assigned to a subscriber being repeated periodically with the frame period duration.
West German Pat. No. DE-OS 25 37 683 discloses a radio transmission system having stationary radio stations and mobile radio stations in which different channel accessing methods with asynchronous time-division multiplex, code-division multiplex and frequency-division multiplex are used.
For codeword synchronisation an incoherent subcarrier demodulation is used. A code generator generates sequentially each of nine different codes, which characterize the stationary ground-based radio stations. After this code has been synchronised with the received signal the IF-signal is multiplied, causing the wide spectrum in the message bandwidth to be transformed. Subsequent thereto the received message can, for example, be recovered using a DPSK demodulator. For the purpose of synchronisation the message code sample, having a length of, for example 15 bits, is used, which precedes the message.
Also combinations of the above-mentioned methods and their use in a digital radio transmission system are known. "Nachrichtentechnik, Elektronik+Telematik 38 (1984), Vol. 7, pages 264 to 268" describes for example a digital radio transmission system in which the time-division multiplex method is used in combination with code distribution. In the time channel for speech and/or data transmission (communication channel TCA) there are sequentially transmitted a bit sequence for determining the bit clock (synchronous), a frame synchronising word (leader) and the bit sequence of the message itself. The time division channels for message transmission (3.times.20 TCA) are combined with the control channels (3 CCH) to form a time-division multiplex frame having a duration of 31.5 msec. If a speech signal is to be transmitted as the message, adaptive delta modulation can be used for analog-to-digital conversion. The message characters (bits) then obtained have a code superposed on them in the transmitter. It has been found to be advantageous to combine individual message characters in blocks of four bits each and to distribute the block thus obtained by means of an orthogonal alphabet. The distribution factor used therewith is a compromise to combine the advantages of band distribution with the requirements as regards economical use of the frequencies. In addition, a message transmission method has been proposed (P 34 47 107.3) in which a different modulation method is utilized in the forward and return directions of the message transmission channels. For message transmission the mobile radio stations access one of a plurality of message channels. In the direction from the stationary radio station to the mobile radio stations assigned thereto, each message channel is distributed by means of code division modulation. The distributed message channels are superposed on each other and the wide-band sum signal thus obtained is transmitted in a common frequency band. In the direction from the mobile radio stations to the stationary radio station the message transmission is effected in separate, narrow-band frequency channels.
For the transmission of speech, in the direction from the stationary radio station to the mobile radio stations, the distribution modulation employed in the mobile radio station is selected by the stationary radio station and reported during the connection setp-up of the mobile radio station. For the transmission of signalling information to the mobile radio station assigned to the stationary radio station a distribution modulation is used which is common to all the mobile radio stations, in the direction from the stationary radio station to the mobile radio stations.
To distinguish between stationary radio stations in adjacent radio cells, these stations transmit, from the stationary radio stations to the mobile radio stations, in different frequency bands. The stationary radio stations include narrow-band receivers which during operation can be switched to a plurality of frequency channels. The number of transmission frequencies switchable in the mobile radio station is less than the number of receiver frequencies switchable in the stationary radio station. It is, for example, possible to effect in the stationary radio station a switch-over to 1,000 frequencies and a switch to 40 frequencies in the mobile radio station.
In each stationary radio station the receiver frequencies used there are managed on the basis of the interference situation. In the case of interferences in the reception, the relevant connection, from the mobile radio station to the stationary radio station, is switched to a different, non-disturbed frequency channel, to which both the stationary radio station and the mobile radio station can switch. The receiver arrangement in the stationary radio station towards the wire network of the public telephone system continues to participate in the connection.
Synchronising the receiver arrangement is very important when block-wise transmission of messages is used, as in the case of incorrect synchronisation the entire block and the message contained therein are mutilated. More specifically, in a radio transmission system in which the connection is effected our propagation paths which are subject to obstruction and in which reflection frequently occurs, errors often occur in the received signal and result in disturbances in the connection. The connection disturbances whose duration and frequency depend on the transmission rate and correspond to a Rayleight distribution, are based on a transmission path-dependent field strength distribution, which in dependence on the reflection coefficients of the environment result in error rates of well over 1%, often substantially 50%.